1. Technical Field
The embodiments described herein relate to a semiconductor integrated circuit and, more particularly, to a voltage generator of a semiconductor integrated circuit.
2. Related Art
A conventional semiconductor integrated circuit uses voltages of various levels. Among them, a pumping voltage (VPP) is generated using an external voltage (VDD) in a voltage generator of the semiconductor integrated circuit.
As shown in FIG. 1, a conventional voltage generator in a semiconductor integrated circuit includes a detector 1, an oscillator 2 and a pump 3. The detector 1 compares a reference voltage (VREF) with a feedback pumping voltage (VPP) to generate an oscillator driving signal ‘OSC_EN’. The oscillator 2 oscillates with the oscillator driving signal ‘OSC_EN’ to generate an oscillation signal ‘OSC’. The pump 3 pumps an external voltage (VDD) in response to the oscillation signal ‘OSC’ to generate the pumping voltage (VPP).
Hereinafter, the operation of a conventional voltage generator will be described.
When the pumping voltage (VPP) is at a low level as compared with the reference voltage (VREF), the detector 1 activates the oscillator driving signal ‘OSC_EN’. However, when the pumping voltage (VPP) is at a high level as compared with the reference voltage (VREF), the detector 1 deactivates the oscillator driving signal ‘OSC_EN’.
During the initial operation of the voltage generator, the pumping voltage (VPP) is at a level lower than that of the reference voltage (VREF). The reference voltage (VREF) is set in proportion to the target level of the pumping voltage (VPP). Since the pumping voltage (VPP) is at a level lower than that of the reference voltage (VREF), the detector 1 activates and outputs the oscillator driving signal ‘OSC_EN’. The oscillator 2 oscillates during the activation period of the oscillator driving signal ‘OSC_EN’ and outputs the oscillation signal ‘OSC’ having a fixed period as shown in FIG. 2. The pump 3 pumps the external voltage (VDD) in response to the oscillation signal ‘OSC’ having a fixed period, thereby outputting the pumping voltage (VPP).
As shown in FIG. 2, since the pump 3 repeatedly turns the pumping operation on/off in response to the oscillation signal ‘OSC’ having a fixed period even after the level of the pumping voltage (VPP) reaches a target level, the pumping voltage (VPP) continuously creates ripple components.
The pumping voltage (VPP) output from the pump 3 is fed back to the detector 1. Accordingly, the pumping voltage (VPP) maintains the target level through the repetition of the aforementioned process.
However, according in a conventional voltage generator, since the pump 3 repeatedly turns the pumping operation on/off in response to the oscillation signal ‘OSC’ having a fixed period, the following problems may occur: First, the pumping voltage (VPP) may not quickly reach the target level. Second, since the pumping voltage (VPP) has many ripple components, the pumping voltage (VPP) becomes unstable.